Bug#30333 (Performance, expressions lists in the parser)

Before this patch, the parser would execute:
- Select->expr_list.push_front()
- Select->expr_list.pop()
when parsing expressions lists, in the following rules:
- udf_expr_list
- expr_list
- ident_list

This is unnecessary, and introduces overhead due to the memory allocations
performed with Select->expr_list

With this patch, this code has been removed.
The list being parsed is maintained in the parser stack instead.

Also, 'udf_expr_list' has been renamed 'opt_udf_expr_list', since this
production can be empty.

sql/sql_lex.h

@@ -596,7 +596,6 @@ public:
   const char *type;               /* type of select for EXPLAIN          */
 
   SQL_LIST order_list;                /* ORDER clause */
-  List<List_item>     expr_list;
   SQL_LIST *gorder_list;
   Item *select_limit, *offset_limit;  /* LIMIT clause parameters */
   // Arrays of pointers to top elements of all_fields list